When pouring molten material into molds it is generally required that during the melting process and during the pouring no products form which may render the casting impure and which may unfavorably influence the structure of the casting, thereby preventing the casting from becoming a true to measurement workpiece upon solidification in the mold. And it is further required that the casting process is performed in an economical manner, preferably as much automated as possible.
In the known casting processes the melting and the pouring is usually done in successive operations, often at different places. For transporting the molten material to the mold the forces of gravity are utilized. During the transport the air can freely influence the material which causes a contamination mainly by the formation of oxides, nitrides and hydrogen. This may considerably lower the quality of the casting. For filling of the molds free from turbulence and for feeding the solidifying casting usually a considerable effort is required in the form of complicated filling and charging apparatuses which often fail to achieve the desired results. Many casting processes have become known which try to overcome these disadvantages. Some of the processes achieve at the same time a high degree of automation. It is also known that for special requirements the metal is molten in the furnace in a vacuum or under the cover of a protective gas. But when the furnace is recharged and when the melt is transported to the mold the influence of air on the melt cannot be avoided under shop conditions.
With the known low-pressure die casting process an almost turbulence-free filling of the mold and dense castings may be achieved with very little molten material needed. It is, however, a disadvantage of this process that the production must be stopped every time the furnace has to be recharged. The latter may be emptied only up to one third of the original charge. Owing to the oxidation of the molten material when being poured freely into the furnace the formation of scum cannot be prevented. For the same reason it is also practically impossible to melt additional solid material. The same dangerous effect occurs also in the rising pipe just below the mold. The effect is caused by the column of molten material which rises and recedes according to the cycles of casting operations. If high quality castings are to be produced the formation of scum must be prevented by inserting a sieving screen between the end of the rising pipe and the mold. It has also been suggested to overcome the disadvantages of the known casting processes by keeping the molten material after the casting operation on the level with the end of the rising pipe. However, this has proved to be very difficult on account of the constantly varying amount of molten material in the furnace. Further, the method has the disadvantage that for each casting operation a considerable amount of compressed gas is needed. For price reasons only compressed air can be used, which, if not carefully dried, may cause a contamination with hydrogen and may also cause the burn-out of such important components as magnesium and sodium. The air volume must be enlarged constantly with the decrease of the molten material in the furnace during the production. Together with the unavoidable thermal expansion of the air this leads to irregular filling conditions of the mold. A further disadvantage rests with the cast-iron rising pipe of which the durability is very limited, depending on the particular material which is molten and cast (see also the German periodical "GieBerei", 1969, Volume 4, page 83-90). The pressure exerted on the melt, which pressure is important for the density of the solidifying casting, must be kept at a low degree because it also effects the inner space of the whole furnace.
Further there are known a number of methods of automatically proportioning the molten metal for the molds or for pressure die casting machines, which methods make use of various valve and pump systems. But they all have the drawback that the air will come into contact with the molten material during the casting process, which results in the above mentioned disadvantages (see also the German periodical "GieBerie", 1962, Volume 8, page 180-189 and Volume 14, page 391-395 and page 400-402).
Further in this connection, there must be mentioned proportioning methods using solid charging material, such as pigs or continuous cast ingots, for displacing molten material out of the furnace. But here again the contact with the air cannot be prevented and, moreover, these methods can only be used with specific types of alloys (see also DBP 1,263,995, further see German periodical "GieBerie", 1968, Volume 25, page 765-768 and volume 18, page 557).
It is therefore an object of the invention to suggest a novel method of melting material and producing high quality castings in an economical manner. It is an other object to suggest a method by which the disadvantages of the prior art methods are overcome. And it is a further object of the invention to provide a furnace for performing the new method.